Réponse des bassins versants à l'activité tectonique : l'exemple de la terminaison orientale de la chaîne pyrénéenne. Approche morphotectonique / Drainage basins response to active tectonics: example from Eastern Pyrenees. Morphotectonic approach

Abstract

Extended Abstract The aim of this paper is to demonstrate the validity of morphométrie analysis in an area of low seismo-tectonic activity. There, drainage divide responds to uplift modifying their morphologic and sedimentologic parameters, such as drainage density, drainage divide size, hypsometric integral value. Tectonic and geomorphic investigations are focused on the relationship between Plio-Pleistocene deformation and landform ruptures using quantitative geomorphology. Morphométrie and geomorphic analysis of drainage basins can be correlated with tectonic input. Correlation research between channel network parameters (such as slope, channel length and drainage density), or basin parameters (area, relief and hypsometry) and uplift is tested. This method is applied to the Tet drainage basin (1500 km2), a Mediterranean coastal river, (eastern Pyrenees, South France). This river was developed during the Neogene time along a major regional fault, NE-SW direction, the Têt fault. This fault is still active (Souriau and Pauchet, 1998). Numerous drainage anomalies, such as antecedence, dissymetry and avulsion, lead us to expect drainage responses to active tectonics. First, Horton's laws are tested. Confluence and length ratio are homogenous in the Tet drainage divide. They reflect the organisation of the drainage network at the regional scale and are not related to local tectonic variations, as previously shown by Kirschner (1993). First order drainage frequency, defined as the number of first order streams per square km, reveals a North - South drainage basin structural asymmetry. The southern part of the drainage basin is excessively dissected. This opposition is confirmed by basin parameter analysis. The sizes of the third and fourth order basins show antagonistic responses at the periphery of the Tet basin. In the southern portion of the basin, the median size of the. third and fourth order basins is 50% greater than their northern counterparts. The same observation can be made about the hypsometric curve. Moreover, this parameter points out the presence of an est - west gradient. On the southern margin of the Têt basin, hypsometric integrals decrease from 0,36 to 0,63. These values are highly correlated with tectonic uplift as deduced from scarp fault morphology, using height, slope and complicity ratios of the vounger spure generation. Morphometric results are compared with field data. The calculation of deformation rates by two different methods shows a linkage between them. During the Pliocene, uplift rates varied from 0.2 mm/year in the eastern part of the fault to 1 mm/year in the western part. The morphostructural organisation of the eastern part of the Pyrenees shows evidence of strike-slip faulting, related to a meridian compressional stress regime. It induced left-lateral faulting on the Tet fault and a propagation of deformation. As a result, the inversion of stress regime in the Pliocene, previously pointed out by other studies (Grellet et al., 1993), is confirmed by a geomorphological approach.